1. Field of the Invention
The present invention relates to an active drive type light emitting display device provided with measuring pixels in addition to light emitting display pixels, and particularly to a light emitting display device and a drive control method thereof in which the light emitting display pixels can be efficiently driven by obtaining forward voltages of light emitting elements by means of the measuring pixels.
2. Description of the Related Art
A display using a display panel which is constructed by arranging light emitting elements in a matrix pattern has been developed widely. As the light emitting element employed in such a display panel, an organic EL (electroluminescent) element in which an organic material is employed in a light emitting layer has attracted attention. This is because of backgrounds one of which is that by employing, in the light emitting layer of an EL element, an organic compound which enables an excellent light emitting characteristic to be expected, a high efficiency and a long life have been achieved which make an EL element satisfactorily practicable.
The organic EL element can be electrically represented by an equivalent circuit as shown in FIG. 1. That is, the organic EL element can be replaced by a structure composed of a parasitic capacitance element Cp and a diode element E which is coupled in parallel to this capacitance element, and the organic EL element has been considered as a capacitor like light emitting element. When a light emission driving voltage is applied to this organic EL element, first, electrical charges corresponding to the electric capacity of this element flow into an electrode as a displacement current and are accumulated. Then, it can be considered that when the voltage exceeds a determined voltage (the light emission threshold voltage=Vth) peculiar to the element in question, current begins to flow from the electrode (anode side of the diode element E) to an organic layer constituting the light emitting layer so that the element emits light at an intensity proportional to this current. FIG. 2 shows light emission static characteristics of such an organic EL element. According to these, the organic EL element emits light at an intensity (L) approximately proportional to a drive current (I) as shown in FIG. 2A and emits light while the current (I) flows drastically when the drive voltage (V) is the light emission threshold voltage (Vth) or higher as shown by the solid line in FIG. 2B. In other words, when the drive voltage is the light emission threshold voltage (Vth) or lower, current rarely flows in the EL element, and the EL element does not emit light. Therefore, the EL element has an intensity characteristic that in a light emission possible region in which the voltage is higher than the threshold voltage (Vth), the greater the value of the voltage (V) applied to the EL element becomes, the higher the light emission intensity (L) of the EL element becomes as shown by the solid line in FIG. 2C.
Meanwhile, it has been known that physical properties of the organic EL element change and its forward voltage (VF) becomes higher due to use over a long period of time. Thus, with respect to the organic EL element, as shown in FIG. 2B, the V-I characteristic changes in the direction shown by the arrow (a characteristic shown by the broken lines) by a real use time, and therefore the intensity characteristic is also deteriorated. The organic EL element also has a problem that variations in initial intensities occur for example also due to variations in deposition at the time of forming a film of the element, whereby it becomes difficult to express an intensity gradation faithful to an input video signal.
Moreover, it has also been known that the intensity characteristic of the organic EL element changes approximately as shown by broken lines in FIG. 2C by temperature. That is, the EL element has a characteristic that in the light emission possible region in which the voltage is higher than the light emission threshold voltage, the greater the value of the voltage (V) applied to the EL element becomes, the higher the light emission intensity (L) thereof becomes, however, the higher the temperature, the lower the light emission threshold voltage becomes. Accordingly, the EL element becomes in a state where light of the EL element can be emitted by a lower applied voltage as the temperature becomes higher, and thus the EL element has a temperature dependency of the intensity that the EL element is brighter at a high temperature and is darker at a lower temperature though the same light emission possible voltage is applied.
In general, a constant current drive is performed for the organic EL element due to the reason that the voltage vs. intensity characteristic is unstable with respect to temperature changes although the current vs. intensity characteristic is stable with respect to temperature changes, the reason that it is necessary to prevent the element being deteriorated by an excess current, and the like. In this case, the drive voltage (VO), which is, for example, brought from a DC/DC converter and the like, supplied to a constant current circuit, has to be set considering respective elements as follows.
That is, as such elements, it is possible to enumerate the forward voltage (VF) of the EL element, a variation part (VB) of the VF of the EL element, a variation part per hour (VL) of the VF, a temperature change part (VT) of the VF, a drop voltage (VD) which is necessary for the constant current circuit performing the constant current operation, and the like. In the case where these elements synergistically affect also, in order that the constant current characteristic of the constant current circuit can be satisfactorily ensured, the drive voltage (VO) has to be set at a value obtained by summing maximum values of respective voltages shown as the respective elements.
However, the case where the voltage value obtained by summing the maximum values of the respective voltages as described above is required as the drive voltage (VO) supplied to the constant current circuit hardly occurs, and in a normal state, a large power loss is caused as a voltage drop part in the constant current circuit. Accordingly, this becomes a main cause of generation of heat, whereby stress is put on the organic EL element, peripheral circuit components, and the like. Japanese Patent Application Laid-Open No. H7-36409 (paragraph 0007 and thereafter and FIG. 1) discloses a structure in which the forward voltage VF of the EL element is measured so that the value of the drive voltage (VO) supplied to the constant current circuit is controlled based on this VF to solve the above-described problems.
The structure disclosed in the Japanese Patent Application Laid-Open No. H7-36409 shows a so-called passive matrix type display device in which respective EL elements are arranged at intersection point positions between respective anode rays and cathode rays. With such a passive matrix type display device, since constant current circuits are equipped for the respective anode rays in the anode drive, it is possible to easily pick up a mean value of the forward voltages VF of the respective EL elements connected to said anode rays by detecting the voltage value of one anode ray.
However, in an active matrix type display device, since active elements constituted by TFTs (thin film transistors) are added to respective EL elements arranged in a matrix pattern to operate the respective EL elements by constant current drive using these TFTs, in order to detect the forward voltages VF of the respective EL elements, it is necessary to draw VF detecting wiring lines from the respective EL elements, for example, from the anode terminals thereof. At this time, in the case of the structure in which drive voltages given to the respective pixels are controlled, for example, by utilizing the forward voltage VF of only one EL element, in the case where a trouble occurs in said EL element for which the forward voltage VF is measured, the entire body including the display panel and module substantially becomes defective. Thus, although a structure can be considered wherein respective VF detecting wiring lines are drawn from a plurality of EL elements so as to measure the mean value of the forward voltages VF of the respective elements, this structure causes physical problems such as a problem that the number of drawn wiring lines increases, whereby realization is difficult.